From mboxrd@z Thu Jan 1 00:00:00 1970 Received: with ECARTIS (v1.0.0; list linux-mips); Sat, 05 Aug 2017 00:25:10 +0200 (CEST) Received: from localhost.localdomain ([127.0.0.1]:38034 "EHLO linux-mips.org" rhost-flags-OK-OK-OK-FAIL) by eddie.linux-mips.org with ESMTP id S23995104AbdHDWZCmj0lx (ORCPT ); Sat, 5 Aug 2017 00:25:02 +0200 Received: from h7.dl5rb.org.uk (localhost [127.0.0.1]) by h7.dl5rb.org.uk (8.15.2/8.14.8) with ESMTP id v74MP1FP008296; Sat, 5 Aug 2017 00:25:01 +0200 Received: (from ralf@localhost) by h7.dl5rb.org.uk (8.15.2/8.15.2/Submit) id v74MP0YD008295; Sat, 5 Aug 2017 00:25:00 +0200 Date: Sat, 5 Aug 2017 00:25:00 +0200 From: Ralf Baechle To: Thomas Petazzoni Cc: linux-mips@linux-mips.org, Waldemar Brodkorb Subject: Re: undefined reference to `__multi3' when building with gcc 7.x Message-ID: <20170804222500.GA11675@linux-mips.org> References: <20170803225547.6caa602b@windsurf.lan> <20170804000556.GC30597@linux-mips.org> <20170804151920.GA11317@linux-mips.org> <20170804174151.2eea9af3@windsurf.lan> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170804174151.2eea9af3@windsurf.lan> User-Agent: Mutt/1.8.3 (2017-05-23) Return-Path: X-Envelope-To: <"|/home/ecartis/ecartis -s linux-mips"> (uid 0) X-Orcpt: rfc822;linux-mips@linux-mips.org Original-Recipient: rfc822;linux-mips@linux-mips.org X-archive-position: 59376 X-ecartis-version: Ecartis v1.0.0 Sender: linux-mips-bounce@linux-mips.org Errors-to: linux-mips-bounce@linux-mips.org X-original-sender: ralf@linux-mips.org Precedence: bulk List-help: List-unsubscribe: List-software: Ecartis version 1.0.0 List-Id: linux-mips X-List-ID: linux-mips List-subscribe: List-owner: List-post: List-archive: X-list: linux-mips On Fri, Aug 04, 2017 at 05:41:51PM +0200, Thomas Petazzoni wrote: > > > > The bug can be reproduced by building with the toolchain available at > > > > http://toolchains.free-electrons.com/downloads/2017.08-rc1-fix-binutils/toolchains/mips64r6el-n32/tarballs/mips64r6el-n32--glibc--bleeding-edge-2017.05-1453-ga703fdd-1.tar.bz2 > > > > and building with the attached kernel configuration file. > > > > > > > > It is not clear to me if this is a kernel issue (lack of __multi3 in > > > > arch/mips/lib/), or a gcc bug in that it shouldn't emit a call to this > > > > function. > > > > > > > > FWIW, sparc64 had a similar issue, and they added __multi3 in their > > > > libgcc replacement, see commit > > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/arch/sparc/lib?id=1b4af13ff2cc6897557bb0b8d9e2fad4fa4d67aa. > > > > > > I think these days we've given up the stuborn resistance of the old days > > > against adding new libgcc1 functions to the kernel, so we should probably > > > just add it. > > > > > > I'm looking into this but a small wrench into the gear is that I'm still > > > on GCC 6 so I'm off to building myself a cross-gcc first ... > > > > I now can reproduce the issue with vanilla FSF binutils 2.28 and GCC 7.1.0. > > Great! However, looking at the functions that end up calling __multi3, > I'm wondering why suddenly gcc 7.x needs to call such a function, while > the same code was compiling without __multi3 in libgcc with gcc 6.x. Chances are it's something specific to MIPS64 R6. Before trying your config file I also tried a number of other defconfigs and all built well. Here's a test case which generates a reference to __multi3: unsigned long func(unsigned long a, unsigned long b) { return a > (~0UL) / b; } GCC rearanges above statement to: return (unsigned __int128)a * (unsigned __int128) b > 0xffffffff; computing which requires 128 bit intermediate results. This is the code generated for MIPS64 R2: dmultu $4,$5 mfhi $2 sltu $2,$0,$2 And this is for R6: move $2,$4 move $7,$5 # $6/$7 contain the second op move $6,$0 move $4,$0 move $5,$2 # $4/$5 contain the first op sd $31,8($sp) balc __multi3 sltu $2,$0,$2 Ralf